Light signals from the environment are perceived by distinct classes of photoreceptors and transduced to the nuclear regulators to control the developmental modes, largely through regulated gene expression. The long-term goal of our research is to dissect the cellular and biochemical basis of this light control of development switch. Our previous work, together with studies from others, has defined a group of evolutionarily conserved COP/DET/FUS proteins that act as light inactivatable repressor of photomorphogenesis. We have established that Arabidopsis COP1 (constitutively photomorphogenic 1) acts within nucleus to suppress the photomorphogenic developmental pathway in darkness and light reverses this repressive action and deplete its nuclear accumulation. Our current NIH funded studies indicates that once within the nucleus, COP1, together with COP10 complex, the COP9 signalosome, and DET1 complex, physically contact and target specific transcription factors (including HY5 and HYH) for ubiquitination and proteasome mediated degradation. The activities of those transcription factors are responsible for promoting photomorphogenic development and gene expression. We also made important progresses in understanding how blue and far red light, once perceived by cryptochromes and phytochrome A, transduced to regulate this switch. Here I propose to combine molecular genetic, biochemical, proteomic, and genomic approaches to comprehensively analyze this regulatory network at the levels of biochemical composition and action, network interactions of the components, and their mediation of light control of genome expression and development. To achieve this goal, I propose three major complementary aspects of further research with eight specific Objectives. The first aspect, including Objectives I to III, is basically to confirm the hypothesized biochemical activities and to reveal the molecular constitutes of the two major players, the COP1 and COP10 complexes. The second aspect, including Objectives IV and VI, is to how far red and blue light signal regulate the COP1 switch through their own photoreceptors and signaling events. The third aspect, including Objectives VII to VIII, is to provide a whole genome level understanding on light control of gene expression and reveal the transcriptional cascade/network at a genomic scale. The accomplishment of these proposed objectives shall provide a comprehensive and mechanistic view of the signaling network responsible for light control of development. This should serve as an essential guidance for investigating similar issues in mammals including human. [unreadable] [unreadable]